Twice I have heard a rather disconerting sound coming from what seems to be the center tunnel between drivers and passengers seats- sounds like a thumping whirring, that occurs on hard acceleration up a slight hill when I am in 3rd maybe 2nd. At first I thought I was running over reflector bumps that were closely spaced... But that's not what it was.

It also seems like the car should downshift, and isn't finding the lower gear.

Is this transmission? What might be the issue?

300sd 1983 stock.

Thanks.